On the hydrocarbon chemistry in a H₂surface wave discharge containing methane

Abstract

Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and four stable molecules, CH4, C2H2, C2H4 and C2H6, in a H2 surface wave discharge (f = 2.45 GHz and power density ≈10-50 W cm-3) containing up to 10% methane under different flows (Φ = 22-385 sccm) and pressures (p = 0.1-4 Torr). The degree of dissociation of the methane precursor varied between 20% and 85% and the methyl radical concentration was found to be in the range of 1012 molecules cm-3. The methyl radical concentration and the concentrations of the stable C-2 hydrocarbons C2H2, C2H4, C2H6, produced in the plasma, increased with an increasing amount of added CH4 as well as with increasing pressure. For the first time, fragmentation rates of methane (RF(CH4) = 1×1015-2.5×1016 molecules J-1) and conversion rates to the measured C-2 hydrocarbons (RC(C2Hy): 5×1013-3×1015 molecules J-1) could be estimated with dependence on the flow and pressure in a surface wave discharge. The influence of diffusion and convection on the spatial distribution of the hydrocarbon concentration in the discharge tube was considered by a simple model.